Hearing aids receive and process acoustic sound to stimulate components of the auditory system to cause the sensation of hearing in a patient. Hearing aids are generally categorized into one of two types, namely, externally worn types and implantable types. In addition, implantable hearing aids can be further categorized into fully implantable devices and semi-implantable, e.g. devices that include some implanted components (typically a signal processor and transducer) and some external components (typically a microphone and speech processor).
One type of implantable hearing aid utilizes a transducer having a vibratory member implanted within the middle ear cavity that mechanically stimulates the ossicular chain via axial vibrations. In one application of such a device, a microphone receives acoustic sound and generates frequency responses for a speech processor. The speech processor, in turn, processes the frequency responses according to internal values for the patient to generate a processed signal that drives the transducer to cause the mechanical stimulation and sensation of sound in the patient.
Unfortunately, over time the frequency responses generated by hearing aid microphones can change, thereby affecting the perception of sound to the patient. The changes in the frequency response can be caused by a number of factors. In semi-implantable and externally worn devices for example, dirt and other debris can collect on or around the microphone port affecting the microphone's frequency responses to acoustic signals. In hearing aids having implanted microphones, changes in the tissue surrounding the microphone can affect the microphones frequency response to acoustic signals. In this case, the changes e.g. thickness, density, and compliance in the tissue, typically occur gradually following the implant and directly affect the sound received in the microphone and thus the resulting frequency response generated by the microphone for the speech processor. The changes in the frequency response can result in either a decrease or increase in the perception of sound to the patient depending on the current state of the tissue. For example, when the microphone is initially implanted and tuned to the patient's hearing needs, the tissue is typically soft. Over time, however, the tissue thickens and a fibrous capsule is formed before a stabilized state is reached. As the tissue changes so does the patient's hearing function, requiring the patient to visit an audiologist for additional tuning of the hearing aid.